Efforts are being undertaken to find strongly dominant trematodes against human schistosomes. Two parasites have been isolated: (1) Echinostoma liei from Bromphalaria alexandrina collected in Egypt and Ethiopia and from B. glabrata collected in Bahia, Brazil, and (2) An Echinoparyphium species from Bulinus truncatus collected in Egypt, Ethiopia, and Yemen Arab Republic. These parasites are being used in interaction studies with human schistosomes. Studies on interaction between trematode larvae of the same species are also being undertaken. Snails already infected with Echinostoma lindoense, E. paraensai, or E. barbosai cannot be reinfected with the same species. However, snails harboring S. mansoni can be reinfected with the same species, although many of the developing mother sporocysts degenerate before they produce daughter sporocysts and only a few reach maturity. Microsporida species found as hyperparasites of trematode larvae in Malaysia are being investigated in the laboratory as potential control agents against human schistosomes.